1. Field of the Invention
The present invention relates generally to the manufacture of metal tubes for use in nuclear fuel rods and, more particularly, is concerned with an automated flush etching, rinsing, drying, inspecting and plugging subsystem and method in a fuel tube manufacturing system.
2. Description of the Prior Art
Because of their critical roles in nuclear reactors, tubes used in fuel rods must meet very stringent quality control standards. Therefore, after formation by multiple pilgering passes, the tubes are subjected to a variety of processing steps including annealing, etching, rinsing, drying, carbide grit blasting, polishing, cleaning and inspecting of the tubes. As practiced heretofore, tube etching, rinsing and drying were carried out on a batch basis meaning that the tubes in lot size bundles, usually six hundred tubes to a bundle (or, in some instances, a half lot size bundle of three hundred tubes) were successively etched, rinsed and dried.
FIG. 1 illustrates a typical prior art tube processing subsystem 10 in a fuel rod tube manufacturing system. To carry out batch etching, as per block 12, a bundle of previously annealed tubes is placed in a carrier with spacers between the tubes. The loaded carrier is transported by a crane to an etching station where it is dipped into an open-top etching tank. Since the opposite ends of the tubes are open, both the inside and outside diameter surfaces of the tubes are etched. To determine whether sufficient material has been removed from the inside and outside tube surfaces by the etching, the entire bundle of tubes must be withdrawn from the etching tank one or more times, inspected by an operator, as per block 14, and then returned to the tank after each withdrawal if more etching is required. Each withdrawal of the bundle from the etching tank exposes the tubes to atmospheric oxygen.
After completion of tube etching, rinsing of the tubes is carried out, as per block 16. The crane transfers the carrier loaded with etched tubes to a rinsing station. At the rinsing station, the bundle of tubes and carrier are lowered into a water tank and the etched tubes rinsed to neutralize any acid residue. The rinsed bundle or tubes is then removed from the water tank and next hosed down while suspended over a drain. The carrier loaded with the rinsed bundle of tubes is then transferred to a drying station, as per block 18, where it is lowered into a forced air chamber. The chamber is closed and a blower and heaters are turned on to force hot air over the bundle to dry the same.
One of the steps following drying of the tubes is ultrasonically testing the tubes for defects, as per block 20. For such testing to be effective, the surfaces of the tubes must be clean and the outside tube surface must be placed in continuous contact with water while the inside of the tube is maintained dry. Since a fluoride layer is now adhered to the tube surfaces due to the performance of etching in the presence of atmospheric oxygen, tube surface cleaning is typically carried out by blasting the surfaces with silicon carbide particles, as per block 22. Then, before ultrasonic testing, the opposite ends of the tubes are sealed by inserting plugs therein, as per block 24. However, before sealing, the tubes are cut to desired length, as per block 26; their outside diameter surfaces are polished or ground, as per block 28; and the surfaces cleaned to remove debris from the cutting and grinding operations, as per block 30. In-process inspections, as per blocks 32-36, accompany each of these steps. Also, immediately before insertion of plugs to seal the tube ends, an inside diameter and straightness inspection is carried out, as per block 38. Then, following completion of ultrasonic testing inspection, the plugs are removed from the ends of the tubes, as per block 40.
In recent times, to improve manufacturing productivity and quality much emphasis has been placed on automation of fuel rod tube manufacture. However, certain aspects of the above-described steps in the conventional tube processing subsystem have tended to constrain such improvements. One aspect is the large number of tubes in each bundle. In processing such a great quantity of tubes concurrently through etching, rinsing and drying, it is cumbersome and time-consuming to have to transport the bundle from one station to the next, and it is difficult to precisely control the quality of individual tubes without substantial manual handling and tube rework being required. Another aspect is etching of the tubes in the presence of atmospheric oxygen. This step has created the problem of fluoride deposits which makes it necessary to introduce extra steps just to rectify this problem. Still another aspect is that cutting the tubes to length before carrying out ultrasonic testing thereof makes it impossible to inspect the tubes completely out to their opposite end edges since the testing equipment recognizes the tube end edges as defects. Thus, the ultrasonic testing equipment must be turned off while the edges pass the sensor which leaves the opposite end portions of the tubes uninspected.
Consequently, a need has evolved for a different approach to certain tube processing steps in fuel rod tube manufacture which promises increased manufacturing efficiency and productivity and improved product quality and reliability.